Linseed oil was epoxided with HCOOOH (formed in situ in reaction of H2O2 with HCOOH) at 35-75 degrees C, stirring rate up to 2000 rpm in presence of H2SO4 catalyst. The reaction course was followed by detn. of epoxy and iodine nos. The highest selectivity of epoxy groups formation (95.2%) was achieved at 45 degrees C, HCOOH:oil molar ratio 3.1:1, H2SO4:oil molar ratio 0.125:1, stirring rate 1500 rpm after 8 h of mixing (double bond conversion 52.1%).